Magnetotellurics
Magnetotellurics (MT) is a geophysical technique invented in the early 1950s, which utilizes naturally-occurring fluctuations of the earth's magnetic field to obtain an image of the earth's subsurface resistivity structure. The resistivity structure can be interpreted in geological terms and converted to an image showing subsurface rock types, thicknesses, structures, etc.
The fluctuations of the magnetic field arise from electric currents flowing in the earth's ionosphere. The changes in these current flows generate low-frequency electromagnetic waves, similar to radio waves, but at much lower frequencies. These waves are propagated around the earth by repeated reflections ("skips") from the surface and the ionosphere. Both surface and ionosphere are far more conductive than the resistive atmosphere in between. The atmosphere is sandwiched between two "mirrors" and thus acts as a "waveguide" or conduit for electromagnetic waves.
These electromagnetic waves have very little curvature since the waves are relatively distant from its source. The situation may be likened to the waves caused by throwing a stone into the surface of a body of water. Close to the point of entry or source, the circular wavefronts are strongly curved. But at greater and greater distances, the circular wavefront becomes less and less curved over a small distance. Taken to the limit, i.e. in "the far field", the wave front can be approximated by a straight line over short distances. When considered in three dimensions, an expanding spherical wavefront can be well approximated by a plane surface over a small region.
One advantage of MT is that the wavefront is almost always far from its source, and is a "plane wave" or equivalently is "in the far field". In practical terms, this means that the signal arriving over a significant geographical area is the same at every location in the area. Again, an analogy from everyday experience is useful: a radio signal arriving on this side of the earth from Australia is experienced as arriving nearly simultaneously, and at the same strength almost everywhere over an area of a few hundred square kilometers.
Unlike radio signals from fixed and definite sources, the MT signals arriving at a specific location can arrive from any direction. The MT signal is thus omnidirectional. The signals from a specific direction may be "well-coupled" or "poorly-coupled" to subsurface targets. The same situation arises with radio antennas. The antenna has to be rotated into the position of maximum sensitivity to the signal from a specific direction. However, in MT, because the signal is omnidirectional, and the orientation of subsurface targets generally unknown, for the best and most general result, it is necessary to use a measuring configuration that is omnidirectional, and thus independent of relative orientations between source-sensors-target.
The "plane electromagnetic waves" arriving at a given measuring area are refracted normal to the surface of the earth. The electromagnetic wave then propagates downward, normal to the surface, losing energy as it does so. The rate of energy loss in the vertical direction is exponential and depends simultaneously on two things: the frequency of the wave, and the electrical resistivity of the earth. The higher the frequency, and the more conductive the earth, the more rapid the attenuation. Thus, a signal at a specific frequency may penetrate quite deeply into the earth where the conductivity is low, or the penetration may be quite limited if the earth is quite conductive.
By Faraday's Law, the changing magnetic fields of the electromagnetic wave cause electric currents to flow in the earth (telluric currents). By measuring at the surface one component of the earth's magnetic field and a component of the earth's electric field at right angles to the magnetic field, it is possible to compute earth resistivity, albeit only in a single direction.
The resistivity of the rocks of the earth's crust varies over a range of more than ten million to one. For example, dense crystalline rocks, such as granite, with little or no pore space and little or no included fluids can have resistivity of approximately 100,000 ohm-m. By contrast, rocks laid down as sediments in ancient oceans frequently preserve in their pore spaces some of the ancient sea water, chemically modified to brines. Brines contain salts, and salty water conducts electricity quite well. Hence, marine sedimentary rocks usually have low resistivities, of a few ohm-m.
In other words, mapping the subsurface resistivity structure of the earth is a type of proxy subsurface geological mapping. Hydrocarbons are usually found in marine sedimentary rocks. Wherever these rocks are covered by denser, more resistive rocks, MT may be used to obtain an image of the subsurface resistivity structure which can be interpreted in terms of the gross rock structure. This is the basic idea behind the use of MT in oil and gas exploration.
Geological facies changes can be strongly correlated with resistivity changes. For example, a sandstone channel (more resistive) may grade into a shale (less resistive). A longshore bar with coarser sediments than its surroundings may preserve this porosity/permeability differential (with corresponding resistivity differential) throughout geologic time. Likewise, near-reef sediments tend to be more resistive then those more distant from the reef, because the sediments near the reef contain a higher proportion of more resistive carbonate fragments derived from the reef by wave action.
High-density networks of MT soundings (3-D MT) can be used to map lateral subsurface conductivity changes within inverted depth ranges that correspond reasonably closely to specific geological units or groups. Plan maps of such lateral resistivity changes (horizontal conductivity gradients) at various depths or in general the horizontal or vertical gradients of any measured or derived MT parameter can be used for a type of proxy subsurface geological mapping, providing information which may assist on locating subsurface oil and gas, geothermal, metal or ground water deposits.
Stratigraphic traps are commonly associated with porosity/permeability boundaries which exhibit significant resistivity changes, Dickey, P. H. and Hunt, J. M. "Geochemical and Hydrogeologic Methods of Prospecting for Stratigraphic Traps" pp. 136-167 in AAPG Memoir No. 16 "STRATIGRAPHIC OIL AND GAS FIELDS--CLASSIFICATION, EXPLORATION METHODS AND CASE HISTORIES" 1972.
Most marine sedimentary rocks contain interstitial brines which exhibit strong variability in ion concentration (and thus strong variability in resistivity) due to variability in fresh water flushing. The saltier the brine, the lower its resistivity. Zones of high salt concentration are sometimes correlated with oil fields. Collins, A. G. "Oilfield Brines" pp. 139 ff in book "Developments in Petroleum Geology - 2" edited by G. D. Hobson, Applied Science Publishers, London, U.K., 1987 and Dickey, P. H. and Hunt, J. M. "Geochemical and Hydrogeologic Methods of Prospecting for Stratigraphic Traps" pp. 136-167 in AAPG Memoir No. 16 "Stratigraphic Oil and Gas Fields--Classification, Exploration Methods and Case Histories" 1972.
The frequently-reported "halo" effect above oil fields is variously ascribed to increased concentrations of metallic magnetic minerals which also have measurable resistivity differences.
Certain shallow heavy oil fields with thick associated brine sections are also suitable targets for resistivity mapping. Klein, J. H. "Spectral Induced Polarization Survey--David Field, Alberta, Canada" presented at 36th Annual Meeting of the Midwest Society of Exploration Geophysicists, Denver, Colo., USA Mar. 6-9, 1983.
It has also been reported that isolated deep reefs have associated geoelectric anomalies, seemingly because of differential compaction and perhaps some other characteristic of the reef structure which exerts a persistent influence on the subsequent deposition through geological time. Yungul, S. H. et al "Telluric anomalies associated with isolated reefs in the Midland Basin, Texas" GEOPHYSICS Vol. 38, June 1973 pp 545 ff.
Anticlinal crests usually exhibit lower resistivity than the surrounding rock because of tensional fractures. Similarly, synformal zones may exhibit higher resistivity due to compression. Overpressured zones are common in rapidly subsiding terranes and the overpressured zone may be a geological significant marker. Overpressured zones have lower resistivities than normally pressured zones due to the greater porosity. Fertl, W. H. "Abnormal Formation Pressures" Elsevier Publishing 1976.
Isolated sandstone lenses in shale formations are often filled with hydrocarbons and encased in overpressured, lower resistivity zones. Fertl, W. H. "Abnormal Formation Pressures" Elsevier Publishing 1976. The brine originally resident in the porous sand has been expelled into the surrounding shale, and oil generated in the surrounding shale has replaced the brine in the pore spaces of the sandstone.
In general any zone of higher porosity and/or permeability, especially in marine sedimentary rocks will exhibit lower resistivity than the surrounding rocks. If the resistivity of the interstitial fluids remains constant, then resistivity will decrease as porosity and permeability increase. Resistivity is especially sensitive to permeability, since zones of high permeability conduct electricity well. High permeability is an especially important property of a good reservoir. Normally, the marine sedimentary rocks can be imaged satisfactorily by seismic or sound waves, originating from man-made explosions or the controlled vibration of specialized machines. The sound waves are reflected from subsurface density boundaries, and the vertical motions caused by reflected waves are sensed at the surface by small, inexpensive units called geophones.
Modern seismic system typically measure the signal on hundreds or even thousands of geophones simultaneously and produce a detailed three dimensional image of the subsurface density structure.
The seismic technique usually produces images of quite high vertical and lateral resolution (a few meters or tens of meters). For this reason, seismic is favored for oil exploration over other geophysical techniques, such as gravity and magnetics, which although are relatively inexpensive, provide a low-resolution image.
MT falls in between, with higher resolution than gravity and magnetics, but significantly higher cost than seismic for comparable a real or linear coverage.
In certain cases, the seismic image may be poor. In locations having a very dense layer in the geological section, the seismic energy is reflected so strongly that little can be seen below these layers. Such dense layers are typically electrically resistive. The MT technique may be used to "see through" the resistive layers and obtain an image of the underlying conductive marine sediments, which seismic cannot see. Although the MT image is of lower resolution than the seismic image, it is still usefull where seismic is poor. This is the main way in which the multinational oil industry has used MT since its inception.
A 1995 MT system typically measures up to 16 input signals simultaneously. In western countries, an MT crew consists of 4 to 5 persons; crew number is usually greater where labor costs less. The equipment usually weighs approximately 300 kg, including a supply of 12 v batteries which are used to power the system. MT is mainly used at isolated points on a grid like or network structure. At each measuring point, three magnetic sensors are installed (to measure three orthogonal components of the magnetic field x-y-z and two wires about 100 m long are laid out in x-y directions and connected to the earth by special non-polarizing electrodes to measure the electric, or telluric field. The 16 channel system can be connected by cables to more than one such measuring site, as well as other outlying sites that measure only the electric field in x-y directions. The motive of the multiple channels is to increase productivity by measuring many channels simultaneously.
The magnetic sensors are immobilized by installing them in shallow trenches; the electric field wires are also restrained from moving as much as possible, since motion of the sensors creates noise signals. The goal is to measure changes of the electric and magnetic fields with stationary sensors. The sensors are connected to an auxiliary unit, called "sensor processor" or "signal conditioner", which filters and amplifies the signals. The amplified signals are then transmitted via a long cable to a "receiver" unit, which contains additional filters and amplifiers. Next the signal is digitized and then stored in memory. The data acquisition proceeds for typically 8 to 16 hours, sometimes longer, in noisy environments, and frequently the equipment operates unattended overnight. The series of digital samples from each of the input channels collected over the entire data acquisition time is termed a "time series".
While the receiver usually is capable of real-time processing of the data, it is usual to transfer the time series data to a central computer for more detailed processing.
This is especially important in very noisy environments. The magnetic and electric signals utilized by the MT technique are quite small. The noise arising from even small motions of the sensors can introduce error into the calculations. Even more serious, modern industrial civilization generates a large amount of electromagnetic signals at an extremely wide range of frequencies, including the low sub-audio frequencies used by MT.
By recording the MT signal simultaneously at two separate locations, some distance apart, it is possible to reduce the noise by processing the data from both locations in a special way. This technique is called "remote reference". The data from each station serves as a reference to the other. Remote reference is possible because the MT signal usually originates some thousands of kilometers from the measuring point and is thus very similar over a distance of many kilometers. The man-made noise or sensor noise usually is correlated over a much shorter distance. Hence the magnetic field is correlated at the two separate sites, whereas the noise is uncorrelated. Depending on the "radius of correlation" of the noise, the two sites may need to be separated by a relatively large distance, perhaps 100 km. In this case, the problem of synchronizing the data acquisition at the two locations has been solved by providing each recording station with a very precise quartz oscillator or "precision clock".
Continuous profiling uses many contiguous in-line individual measurements of the electric field and combines with simultaneous measurements of at least 2 orthogonal horizontal components of the magnetic field provides resistivity soundings at all electric field measuring points. The measuring line is kept as straight as possible. The contiguous electric field measurements can be summed together in suitably weighted fashion, and in suitably increasing quantities (i.e. increasing lengths) to provide the basis of a spatial filtering technique which smooths out the effect of localized surface resistivity variations. Such near-surface resistivity inhomogeneities can distort the electric field measurement, causing errors in the calculation of the earth's resistivity. This phenomenon is known as static shift and it is a serious problem in interpretation of MT data. The continuous profiling technique is described in U.S. Pat. Nos. 4,591,791; 4,757,262; and 4,835,473 as well as the article "Principles of spatial surface electric field filtering in magnetotellurics: Electromagnetic array profiling (EMAP, by C. Torres-Verdin and F. X. Bostick, GEOPHYSICS Vol. 57, pp. 603-622, 1992. On page 608 of this article it is clearly brought out that simultaneous synchronized data acquisition at a very large number of MT measuring stations is beyond the then state of the art.
Continuous profiling and spatial filtering (i.e. smoothing) was originally promoted as a solution for the problem of static shift. However, like all smoothing techniques, it distributes error rather than removing it. Hence it cannot remove static shift. However, it can smooth its effects and thus provide a subsurface conductivity image with less local variation which is considered to be easier to interpret. In addition, the continuous image is easier for the human eye to visualize and interpret and provides higher spatial resolution than isolated point soundings. These advantages have led to an increase in the use of continuous MT profiling in recent years.
A little-mentioned disadvantage of the continuous profiling technique is that the in-line measurement of a single component of the electric field permits of only a scalar interpretation at the single-component electric field measuring points. The disadvantages of the scalar measurement are well known to practitioners of the art.
A more serious disadvantage of the continuous profiling technique is that it imposes an electric field measurement length of typically 200 m which is absolutely required by the spatial filtering algorithm which characterizes the technique, in order to define the near-surface layers, but which is actually unnecessary for lateral resolution of deeper targets. In other words, the continuous profiling "solution" actually creates extra cost and guarantees a practical limitation to the application in the field. Because the MT signal is omnidirectional, each MT sounding point has a lateral zone of sensitivity which may be likened to an inverted, truncated cone. The continuous profiling technique must "oversample" the electric field. Oversampling leads to higher cost, as well as logistic penalties when it is difficult to maintain the continuity of the line.
The continuous profiling technique requires continuous, contiguous in-line measurements of the electric field. As a result, the technique is vulnerable to loss of one or more points. This can arise from several causes which are quite common in real-world exploration.
Moreover, the continuous profiling technique is poorly adapted to wide-area reconnaissance. For example, it is a typical feature of oil and gas or other resource exploration license blocks that significant areas of the block have to be relinquished after each stage of exploration.